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Insulating Foundations

An uninsulated foundation may account for up to 50% of the heat lost from an otherwise tightly sealed, well-insulated house. One way to combat this and make above-grade spaces more comfortable, is to insulate the foundation. This can also be a low cost way to increase a home's useful living space.

Proper installation of foundation insulation material is necessary to avoid moisture condensation, material damage, and structural decay caused by the difference in temperature between the house interior and the adjacent earth. Poor design and installation may also aggravate radon infiltration and insect infestation.

How you insulate your foundation depends on a number of factors. Is it new construction or retrofit? Does the house have a deep basement, shallow basement, crawl space, or slab-on-grade? Is the shape (or plan) simple or complex? What type of insulation is best? Are you planning to heat or cool the lower level? Do you plan to use radiant floor heating? These are a few of the factors you must consider.

Base the amount of insulation necessary for foundations on the climate, soil type, size of the house, type of foundation, fuel costs, and the project budget. For example, foundations in soils that retain moisture (such as clay), which can saturate the insulation, need higher levels of insulation. The Building Foundation Design Handbook (see Bibliography) lists the recommended amount of insulation for different types of foundations.

Insulating Basement Walls

Installing insulation on the exterior of a basement wall ("exterior insulation") is usually good practice. New construction and retrofit of existing buildings commonly use exterior insulation.

Exterior insulation has the following advantages and disadvantages:

Advantages:

it minimizes thermal bridging and reduces heat loss through the foundation,

it protects waterproofing,

it can serve as a capillary break to moisture intrusion,

it prevents freeze-thaw cycle damage to the foundation,

it reduces interior moisture, and,

in a retrofit, it does not reduce usable interior space.

 

Disadvantages:

installation is much more difficult in retrofits than interior installation,

material cost is high, and

many exterior insulation materials are susceptible to insect infestation.

Insects using exterior insulation as a pathway into the house structure has recently alarmed both building owners and regulatory groups. The U.S. Department of Energy is working with regulatory groups to help establish appropriate guidelines that provide cost-effective thermal protection for buildings.

Building scientists theorize that the best way to build a dry basement is to insulate the outside of exterior walls with a rigid, fibrous, insulating drainage layer, such as fiberglass or rockwool, and omit the common application of exterior dampproof coating or interior vapor retarder. Apply conventional dampproofing to the upper (within 3 feet [1 meter (m)] of grade), below grade portion of the wall. The fibrous insulation acts as a capillary break that keeps bulk water out even during floods. The concrete will always dry to the exterior due to the vapor pressure differentials. This construction resists summer wall condensation and can potentially act as a passive dehumidifier for the basement. In winter, water vapor will diffuse inward whenever the relative humidity of the basement air is below 33%.

Foam insulation impregnated with insecticidal boric acid has yielded some success in discouraging termite infestations. Although termites avoid it, boric acid slowly leaches out of most materials exposed to moisture. Installation of a good gravel or manufactured "rain screen" drainage element outside the insulation can significantly reduce moisture problems and structurally protect the insulation.

Insulation may also be applied to the interior of the foundation or basement wall. This is popular for retrofit situations. Interior insulation has the following advantages and disadvantages:

Advantages:

it is simpler to install on existing foundation walls, and

material costs may be low since you may use almost any insulation material.

 

Disadvantages:

many types of insulation require separation from habitable spaces by a fire-rated material since they are often extremely flammable and release toxic gases when ignited,

it reduces usable interior space particularly when retrofitted,

it fails to protect the waterproofing or structure as does exterior insulation, and

it may become saturated by moisture.

Proper installation of sealants and vapor diffusion retarders are important for adequate performance of interior insulation.

Insulation materials may be placed inside existing structural cavities in retrofits, whereas new construction lends itself to the additional use of structurally integrated insulation materials. Rigid foam applied in new construction may be placed in the middle of a cast-in-place concrete wall. It may also serve as the permanent inner and outer form faces either in panel form or interlocking rigid foam units.

Cavity foundation materials, such as concrete block, potentially lend themselves to both retrofit and new construction installations of foamed-in, blown-in, and poured-in insulations. The most commonly used poured-in materials include polystyrene beads and granular materials such as vermiculite. Foamed-in insulations sometimes are more appropriate. Blown-in insulation is rarely used to fill concrete blocks.

Filling the cavities of hollow masonry requires no additional framing or wall finishing. Although it reduces convection within the hollow cavity, significant levels of heat can still conduct through the webs of the masonry. Concrete block is available with insulating inserts for new construction. Some concrete block manufacturers attempt to increase the thermal resistance of their product by adding materials such as polystyrene or wood chips to the concrete mix.

Insulating Slab-On-Grade Foundations

To insulate new slab-on-grade foundations, or foundations for homes that have no basements, use one of the following methods of insulation placement. Place the insulation vertically on the exterior or the interior of the foundation wall, or horizontally above or below the floor slab. Each approach has its advantages and disadvantages.

Continuous vertical exterior insulation placed outside the foundation wall reduces heat loss from both the foundation and the slab. Insulate any exposed slab edge above grade. To reduce heat flow from the slab floor to the ground outside, extend the insulation below grade to the footing. Cover the insulation with a protective membrane, the type of which will depend on if the insulation is above or below grade.

Before pouring the slab, install insulation vertically on the interior side of the foundation. This is sometimes easier than exterior applications for slab-on-grade foundations. The Building Foundation Design Handbook (see Bibliography below) is an excellent reference for properly detailing this and other foundation and slab insulation conditions.

Insulating horizontally below an existing slab-on-grade foundation is usually impractical. Horizontal insulation installed below a floor slab in new construction consists of the following cross-section:

floor slab (top),

2 to 3 inches (51 to 76 millimeters [mm]) of damp sand fill,

appropriate rigid insulation (usually 1 to 2 inches [25-51mm] in thickness),

moisture retarder (usually one layer of 6 mil [0.006 inch; 0.15mm] polyethylene), and a

sand or washed gravel drainage layer (usually 4 inches [102mm] deep).

Insulation may be applied on top of the existing slab as described in the following floor cross-section:

finish flooring (top),

rosin paper,

subflooring, and

insulation (usually rigid foam) installed on top of a moisture barrier (6 mil [0.006 inch; 0.15mm] polyethylene) laid across both moisture resistant furring strips (laid parallel) and the floor slab.

You may place a floating wood floor directly on the rigid foam board insulation without sleepers.

Horizontal installations have the following advantages and disadvantages:

Advantages:

it is a simple installation for retrofit work,

it can thermally isolate the building if continuous with vertical insulation, and

it yields floor surfaces at approximately the ambient interior air temperature.

 

Disadvantages:

it can be a fire hazard requiring special protection,

it may encourage increased frost depth around the slab edge, and

it separates the space above from the temperature moderating effects of the earth.

Due to the complex nature of thermal mass transfer, general guidelines are difficult to outline. In most climates, the annual heat captured within the house creates a zone of warm earth directly under the house. This moderates the house temperature, particularly in the lower level. If the entire building is isolated from the earth by insulation, this does not occur.

Insulating Crawl Spaces

How to insulate a crawl space depends on whether you vent it. Traditionally, crawl spaces have been vented to prevent problems with moisture. Place insulation for vented crawl spaces between the overhead joists, tight against the subfloor. In regions with 20 inches (508 mm) or more of annual precipitation, place a 6 mil (0.006 inch [0.15 mm]) polyethylene vapor retarder, or equivalent material for ground cover, over the floor of the crawl space. Fiber-reinforced polyethylene resists tears that quickly destroy standard unreinforced polyethylene. Covering the polyethylene with a thin layer of damp sand and a 2 inch (51mm) "mud" slab of concrete is a good way to protect the vapor retarder from degradation and improve maintenance access. Be sure the minimum finished slab-to-structure clearance conforms with local code regulations before building up the crawl space floor with sand and mud slab.

If the crawl space is unvented, install rigid insulation against the inside or outside of the crawl space walls. Another common, low-cost approach is to insulate the crawl space by draping fiberglass batts over the inside face of the foundation walls. This application requires a vapor retarder. For more information on installing the vapor retarder, consult the Building Foundation Design Handbook (see Bibliography).

Installation Cost and Performance

Although you can achieve considerable savings in heating costs by insulating the foundation, installation costs often become high, particularly for retrofit projects. The materials used, the location of the application, and the extent and timing of the work all affect the overall cost. While there are savings accrued through energy use reductions, "energy savers" financing packages, and home improvement savings programs, simple payback is typically the deciding analysis method. The payback period can range from 6 months for a simple do-it-yourself installation to 20 years for more involved work.

The economics of foundation insulation for new house construction are much more favorable, since installing insulation during initial construction is less expensive. Although measured data is scarce, computer modeling studies have estimated the cost-effectiveness of foundation insulation in new residential buildings in many U.S. cities. Except in the warmest climates, calculations indicate that adding some foundation insulation in new residential construction is cost-effective.

More comprehensive analysis is needed to better identify appropriate protective coatings, address insulation moisture absorption, and understand long-term insulation R-value degradation. One study conducted by the Minnesota Department of Public Service, Office of Energy Conservation, surveyed 59 houses in the Minneapolis-St. Paul area from April to June of 1988. The study sampled foundation insulation specimens and soil specimens to determine long-term performance.

The survey's results showed that the durability and performance of exterior foundations are due to installation quality and above-grade protective coatings, rather than the type of insulation material used. Most coatings help to minimize moisture absorption and foster R-value retention. However, almost 60% of the bitumen coatings (commonly used to protect spray urethane insulation) sampled showed flaking, gouging, or other damage that could reduce effectiveness. For a more detailed discussion of the findings of the Minnesota study, refer to the April 1989 issue of Energy Design Update and the May/June 1989 issue of Home Energy (see Bibliography).

Proper insulation of the foundation can potentially reduce energy costs and create a more comfortable home. It is important, therefore, to plan the installation based on sound principles directly applicable to your site and design. The references in the Bibliography have additional information on techniques for insulating foundations.

Bibliography

Books, Reports, and Conference Papers

Building Foundation Design Handbook, K. Labs, J. Carmody, and R. Sterling, Underground Space Center, University of Minnesota, 1988. Available from National Technical Information Service (NTIS) (see Source List). 349 pp., $61.50 (softcover), Order No. DE88013350.

"Crawl Spaces: Regulations, Research and Results," W. Rose, Bugs, Mold & Rot II: Proceedings of the Moisture Control Workshop, Washington, DC, November 16, 1993, pp. 83-88. Available from the National Institute of Building Sciences (NIBS), Publications Department, 1201 L Street, NW, Suite 400, Washington, DC 20005, (202) 289-7800, Fax: (202) 289-1092, Internet: (E-mail) nibs@nibs.org. $35.00 (full proceedings).

"Dry Basements through the Selective Use of Thermal Insulation and Moisture- Resistant Materials," J. Timusk, K. Pressnail, and W. Chisholm, The 1995 Excellence in Housing Conference: Innovations for Performance, Minneapolis, MN, March 8, 1995, pp. A57-87. Available from the Energy Efficient Building Association (EEBA), Inc. (see Source List). $15.00 (reprint), $50.00 (full proceedings).

Design Guide for Frost-Protected Shallow Foundations, National Association of Home Builders (NAHB), Research Center for the U.S. Department of Housing and Urban Development (HUD), 1994 . Available from HUD User, P.O. Box 6091, Rockville, MD 20849, (800) 245-2691 or (301) 251-5154. 48 pp., $5.00 (softcover), HUD User No. HUD6507. The Design Guide can also be downloaded off the Internet as a Gopher file (huduser.aspensys.com 73). The downloaded version is free.

Design Guide for Frost-Protected Shallow Foundations (2nd Ed.), NAHB Research Center for the U.S. Department of Energy (DOE), 1996 . Available from the NAHB Research Center, 400 Prince Georges Center Boulevard, Upper Marlboro, MD 20772- 8731, (301) 249-4000. 48 pp., $29.00 (softcover).

"Insulating Building Foundations for Frost Protection, Energy Conservation, and Affordability," J. Crandell, P.E., The 1994 EEBA Conference: Excellence in Housing, Dallas, TX, February 23, 1994, pp. A106-22. Available from EEBA, Inc. (see Source List). $15.00 (reprint), $45.00 (full proceedings).

Moisture Control Handbook, J. Lstiburek and J. Carmody, Dames and Moore, Trow Inc., 1991. Available from NTIS (see Source List). 247 pp., $55.00, NTIS Order No. DE92002388, Report Number ORNL/Sub/89-SD350.

Moisture Control Handbook—Principles and Practices for Residential & Small Commercial Buildings, J. Lstiburek and J. Carmody, Van Nostrand Reinhold, 1993. Available from Thomson Publishing Education Group Distribution Center, 7625 Empire Drive, Florence, KY 41042, (800) 354-9706. $49.95 (hardcover), ISBN 0- 442-01432-5.

The Portland Cement Association's Guide to Concrete Homebuilding Systems, P. VanderWerf and W. Munsell, McGraw Hill, 1995. Available from the Portland Cement Association (PCA) (see Source List). 300 pp., $35.00 (hardcover), PCA order no. SP-205, ISBN 0-07-067020-X.

The Portland Cement Association's Insulating Concrete Forms Construction Manual, P. VanderWerf and W. Munsell, McGraw Hill, 1996. Available from the PCA (see Source List). $27.00 (softcover), $25.00 (video, 29 min.), $45.00 (combined).

"Thermal Performance of Concrete Masonry Unit Wall Systems," J. Kosny, Oak Ridge National Laboratory, 1995 . Available from NTIS (see Source List) . 20 pp., $19.50 (papercopy), NTIS order number DE96005466.

Articles

"Avoiding Foundation Failures," R. Marshall, Journal of Light Construction, (14:10) pp. 33-36, July 1996.

"Beadboard Below Grade," J. Nisson, Energy Design Update, (15:11) p. 9, November 1995.

"Breathing Basement Walls," J. Nisson, Energy Design Update, (15:5) pp. 6-7, June 1995.

"On Bugs in Foam Foundation Insulation," J. Nisson, Energy Design Update, (16:1) pp. 4-6, January 1996.

"CABO Rejects Cement Association Proposal on Frost-Protected Shallow Foundations," J. Nisson, Energy Design Update, (15:11) pp. 2-3, November 1995.

"Cautious Against Undermining Basement Wall," B. Juedes, Fine Homebuilding, (No. 97) p. 8, August/September 1995.

"A Complete Guide to Insulating Foam Concrete Form Systems," J. Nisson, Energy Design Update, (16:2) pp. 13-15, February 1996.

"Concrete Sealing Products," Fine Homebuilding, (No. 98) p. 18, October/November 1995.

"Design and Savings for Foundation Insulation," J. Nisson, Energy Design Update, (7:7) pp. 14-15, July 1988.

"Foam Attachment System for Foundation Walls," J. Nisson, Energy Design Update, (15:7) p. 13, July 1995.

"Foundation Insulation—Part 1," A. Wilson, New England Builder, (4:6) pp. 33-34, March 1986.

"Foundation Insulation—Part 2," A. Wilson, New England Builder, (4:7) pp.40-41, April 1986.

"Foundation Insulation—Part 3," A. Wilson, New England Builder, (4:8) pp. 35-36, May 1986.

"Foundation Summary," K. Labs, Northeast Sun, (6:1) pp. 6-9, February 1988.

"Foundation Vision Strip Is Energy Loser," Journal of Light Construction, (14:8) p. 10, May 1996.

"In-Service Performance of Exterior Foundation Insulation," J. Nisson, Energy Design Update, (8:4) pp. 2-3, April 1989.

"Insulated Slabs: Details and Practices," K. Labs, Journal of Light Construction, (7:6) pp. 58-59, March 1989.

"Insulating Basements and Slabs," R. Ruggles, Southface Journal of Energy and Building Technology, pp. 16-17, Winter 1988.

"Insulation Levels for Foundations," J. Christian, Progressive Builder, (6:6) pp. 11-18, June 1987.

"Mineral Fiber Foundation Insulation and Drainage Board—Move Over Styrofoam?" J. Nisson, Energy Design Update, (15:8) pp. 12-13, August 1995.

"Moisture in a Walk-Out Basement," J. Ponessa, Fine Homebuilding, (No. 98) pp. 14, 16, 18, October/November 1995.

"A Proper Foundation?" Home Energy, (12:2) pp. 3-4, March/April 1995.

"Oak Ridge Will Test Thermal Mass Benefits of Insulating Concrete Forms," J. Nisson, Energy Design Update, (15:10) p. 3, October 1996.

"Operation Foundation," J. Gunther, Popular Science, (247:6) p. 31, December 1995.

"Optimal Slab-on-Grade Foundation Insulation," J. Carmody, Northeast Sun, (6:3) pp. 16-17, June 1988.

"Precoated Foundation Insulation Panels," J. Nisson, Energy Design Update, (8:5) pp. 10-11, May 1989.

"Simplified Rigid Foam Basement Insulation System," J. Nisson, Energy Design Update, (15:11) p. 13, November 1995.

"Should You Insulate the Basement?" J. Nisson, Journal of Light Construction, (10:9) pp. 44-46, June 1992.

"To Insulate a Basement," M. Quaid, Home Energy, (6:3) pp. 12-16, May/June 1989.

"Termites in Foam Foundation Insulation—An Update," J. Nisson, Energy Design Update, (15:11) pp. 7-8, November 1995.

"The Thermally Sound Basement," K. Labs, Solar Age, (10:1) pp. 24-26, January 1985.

"Views on Insulating Foundation Walls," C. Silver and T. Brennan, Progressive Builder, (11:7) pp. 41-43, August 1986.

Source List

Energy Efficient Building Association, Inc.
2950 Metro Drive, Suite 108
Minneapolis, MN 55425-1898
Phone: (612) 851-9940; Fax: (612) 851-9507
Internet: (E-mail) EEBA@aol.com;
(World Wide Web) http://www.eeba.org/

Insulating Concrete Form Association
Dick Whitaker, Executive Director
960 Harlem Avenue, Suite 1128
Glenview, IL 60025
Phone: (708) 657-9730; Fax: (708) 657-9728

National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Phone: (800) 553-6847 or (703) 487-4650; Fax: (703) 321-8547
Internet: (E-mail) orders@ntis.fedworld.gov;
(World Wide Web) http://www.ntis.gov

Portland Cement Association
5420 Old Orchard Road
Skokie, IL 600077
Phone: (708) 966-6200; Fax: (708) 966-8389

EREC is operated by NCI Information Systems, Inc. for the National Renewable Energy Laboratory/U.S. Department of Energy. The statements contained herein are based on information known to EREC at the time of printing. No recommendations or endorsement of any product or service is implied if mentioned by EREC.

Energy Efficiency and Renewable Energy Clearinghouse (EREC)
P.O. Box 3048 Merrifield, VA 22116
Voice: 1-800-DOE-EREC
E-mail: doe.erec@nciinc.com
BBS: 1-800-273-2955

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